One of the major potential disadvantages to asexual reproduction is believed to be a reduction in phenotypic variability. This study represents an empirical test for such a reduction in the variance of physiological traits in parthenogenetic species of the lizard genus Cnemidophorus. Five performance traits (burst speed, endurance, maximal exertion, standard metabolic rate and evaporative water loss rate) were examined in four asexual species and the sexual species that hybridized to produce them. A phylogenetically controlled analysis revealed less trait variance in asexual species for the first three traits, but no detectable differences between asexual and sexual species for the other two traits. A second analysis examining the average shape of trait distributions in the two types of species suggests that sexual populations produce distributions with more elongate tails than do asexual populations. Thus, part of the reason for increased variance in sexual populations may be a greater tendency for these populations to produce extreme phenotypes.

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